1. Field of Invention
This invention relates to reverse thermal gels. More specifically, this invention relates to heparin-mimicking sulfonated reverse thermal gel as a protein delivery system created by the sulfonation of a graft copolymer.
2. Brief Description of the Related Art
The development of smart, or stimuli-responsive, biomaterials is a major research focus in the field of tissue engineering and biomolecule delivery. Temperature-responsive reverse thermal gels (RTG) are a group of stimuli-responsive biomaterials that have gained much attention in recent investigations. At room temperature, RTG systems exist in a solution state (sol) with low viscosity that allows injection through a small gauge needle. Upon reaching body temperature, the RTG transitions from a low-viscosity sol to a semi-solid gel (sol-gel phase transition). This unique characteristic may be used to facilitate the delivery and subsequent release of sensitive therapeutic agents, such as cells, drugs or proteins, to a specific target site, avoiding side effects of invasive surgeries. In particular, an RTG can exist as a mobile viscous liquid at low temperatures, but RTGs become a more rigid semisolid gel at higher temperatures. By controlling the composition of the RTG, it is possible to use these polymers to design a formulation that is liquid at room temperature, but gels once injected. The RTG can then function as a depot of a drug at the injection site.
Heparin is a naturally sulfated biopolymer with an intrinsic negative charge. Heparin stores, protects and stabilizes positively charged heparin-binding proteins in the extracellular matrix (ECM) and plays an important role in the regulation of cellular proliferation and differentiation. Due to heparin's ability to associate with positively charged heparin-binding proteins, a vast number of heparin-conjugated protein delivery systems have been investigated. However, heparin has major limitations. It is difficult to modify, susceptible to desulfation, and presents batch-to-batch variability in structure and biocompatibility. It also has significant undesirable activity in other non-target biological pathways [Kanabar V, et al., British Journal of Pharmacology 2008; 154(4):833-842]. Moreover, it may inhibit the normal growth of certain cells, such as human umbilical vein endothelial cells and human dermal fibroblasts [Ferrao A V, et al., Biochimica et Biophysica Acta (BBA)—Molecular Basis of Disease 1993; 1180(3):225-230; Cariou R, et al., Cell Biology International Reports 1988; 12(12):1037-1047].
Sulfonation, or sulfation, of polymeric materials may induce a biofunction similar to that of heparin [Nguyen T H, et al., Nature Chemistry 2013; 5(3):221-227; Guan R, et al., Bioconjugate Chemistry 2004; 15(1):145-151; Liekens S, et al., Molecular Pharmacology 1999; 56(1):204-213; Sangaj N, et al., Biomacromolecules 2010; 11(12):3294-3300]. However, the nature of these bulk-sized scaffolds may still require invasive surgeries for implantation.
A system that mimics the biofunction of heparin and delivers therapeutic proteins non-invasively to a zone of interest would be an ideal alternative to previous delivery systems. Accordingly, it is an object of the present invention to provide a delivery system for positively charged therapeutic proteins and similar biologically active compounds that overcomes the limitations of prior systems, including the adverse side effects associated with heparin. It is a further object of the invention to provide heparin mimicking delivery system that performs as a reverse thermal gel at physiologically relevant temperatures. As will become apparent in the following disclosure, the present invention meets these important needs.